Thatch is either straw or, better and longer lasting, reed.
The colour is probably closer to straw-yellow when brand new and as it ages it becomes darker, browner and eventually can have a silver-grey colour. How it is applied and its thickness and density varies with the material available and how expensive the thatch material is. Olive drab is not typical from my experience, but 'sod roofs' made of living turf are usually shades of green. I have seen some images, in black and white, of peasant huts in central and eastern Europe where the thatch has obviously been patched with new areas of cleaner, brighter straw or reed. However, houses are usually thatched in one go and the colour tends to be fairly homogeneous with little major variation. Here in England the weather tends to darken the thatch quite rapidly and its yellow hue fades to buff and then brown quite rapidly. The ridge is usually thicker and often decorative shapes are cut into it.
Try this site:
http://commons.wikimedia.org/wiki/Category:Thatched_roofs
to get some ideas. Do an internet search and look at sites from around the world-there is wide variation, but the colour and style of thatch depends upon the sort of building you are modelling and where it is located as well as age. No matter what you do someone somewhere will dispute the type and colour as being only ever found in Lower Saxony, South West Norfolk or even downtown Manhattan!
Best wishes,
John
